Scale inhibitors for preventing or reducing calcium phosphate and other scales

ABSTRACT

Acrylic acid or acrylic acid/methacrylic acid is polymerized with between 5-95 mole percent of (meth)acryloyl morpholine to form polymers having a molecular weight range between 1,000-150,000. These polymers are extremely effective inhibitors for preventing calcium phosphate scale in boilers and on industrial heat exchangers. The above described water-soluble polymers are novel.

INTRODUCTION

Certain boiler waters and many industrial waters such as those used inthe operation of cooling towers are treated with a variety of inorganicand organic phosphorous-containing compounds. Such treatments tend toproduce calcium phosphate scales which adhere to the metal surfaces ofboilers and metallic heat exchangers.

Many of the known organic scale inhibitors and scale dispersants, bothinorganic and those containing water-soluble polymers, while beingeffective against a wide variety of scales, are not entirely effectiveagainst calcium phosphate scales.

Pure calcium phosphate scales may exist as such but frequently are foundas contaminants of calcium carbonate and calcium or magnesium saltscales. When such scales contain at least 10% of calcium phosphate, theyare suitable for treatment with the scale inhibitors of the invention aswill be more hereinafter defined.

THE INVENTION

A method of inhibiting scales generally and specifically and preferablycalcium phosphate scale in boilers and industrial cooling systems whichcomprises treating the water present in such systems with a few ppm of aco- or terpolymer of (meth)acryloyl morpholine. The comonomer is acarboxylate-containing vinyl monomer. Examples of the comonomers areacrylic acid, methacrylic acid, maleic acid, crotonic acid, isocrotonicacid and itaconic acid. The termonomer can be any vinyl monomer as longas the resulting terpolymer is water-soluble. Examples of thetermonomers are methacrylic acid, maleic acid, itaconic acid, vinylacetate, vinyl sulfonic acid, AMPS, acrylamide, N-alkanol acrylamide,N-alkyl acrylamide, methacrylamide, N-alkyl methacrylamide,methylacrylate, ethylacrylate, propyl acrylate, 2-hydroxyethyl acrylateand 2-hydroxy propyl acrylate. These polymers contain 5-95 mole percentof acryloyl morpholine and have a molecular weight within the range of1,000-150,000.

In a preferred embodiment of the invention, the polymers contain between10-30 mole percent of acryloyl morpholine. They preferably have amolecular weight range between 3,000-100,000.

The dosage necessary to inhibit calcium phosphate scale will vary fromas little as a few ppm up to as much as 20 ppm or more. In many casesabout 10 ppm gives good calcium phosphate scale inhibition. The ppmdosage is based upon the ppm of calcium phosphate contained in thescale-forming waters to be treated.

The exact mechanism by which the polymers of the invention work is notfully understood although they are believed to act as inhibitors ofcrystal growth as well as dispersants for fine particles of calciumphosphate present in industrial boilers or industrial cooling waters.

The invention also contemplates the above described water-solublepolymers as being novel compositions.

EXAMPLES

To illustrate the invention, the following are given by way of example:

Preparation of the Polymers EXAMPLE 1 Acrylic acid (70 mole %)/AcryloylMorpholine (30 mole %) Copolymer

A solution of acrylic acid (50.4 g), acryloyl morpholine (42.3 g), waterand 50% NaOH (177.3 g) with a pH of 4.5 was charged into a 1.5 literautoclave. The solution was heated to 124° F. in the presence of air.Ammonium persulfate (1.63 g) in water (10 g) and sodium bisulfite (4 g)in water (20 g) were added to the solution in sequence and the autoclavevalves were quickly closed. Temperature of the reaction went up to 228°F. in 0.6 minutes and the polymerization was completed in 7 minutes.

G.C. analysis showed the sample contained 6700 ppm acrylic acid and 3900ppm acryloyl morpholine. The molecular weight of the copolymer was12,000 as determined by GPC method using polystyrene sulfonic acidstandard.

EXAMPLE 2 Acrylic Acid/Acryloyl Morpholine Copolymer

A solution of acrylic acid (40.28 g), acryloyl morpholine (19.72 g) inwater (340 g) was charged into a 1-liter, 4-neck round bottom reactionflask which was equipped with a mechanical stirrer, a thermomenter, anda condenser. The solution was heated to 60° C. under nitrogenatmosphere. Ammonium persulfate (3 g) and sodium bisulfite (9 g) wereadded in sequence. Reaction temperature went up to 90° C. and graduallycooled to and maintained at 60° C. for two hours. At the end of thereaction, a small amount of gel was separated from solution and wasseparated.

The samples analyzed by G.C., G.P.C. and C-13 NMR were found to contain450 ppm residual acrylic acid, 410 ppm residual acryloyl morpholine; themolecular weight of the copolymer was 11,200, and the composition of thecopolymer was about 90 mole % acrylic acid and 10 mole % acryloylmorpholine.

EXAMPLE 3 Acrylic Acid (70 mole %)/Acryloyl Morpholine (30 mole %)Copolymer

A solution of acrylic acid (34.80 g), acryloyl morpholine (29.20 g), 50%sodium hydroxide and water (330.11 g) with a pH of 4.5 was charged intoa 1-liter, 4-neck round bottom flask which was equipped with amechanical stirrer, a thermometer, and a condenser. The solution washeated to 60° C. under nitrogen amosphere and ammonium persulfate (0.64g) in water (10 g) and sodium bisulfite (1.92 g) in water (20 g) wereadded in sequence. The reaction was kept at about 65° C. for five hours.

The molecular weight of the copolymer was 90,100 as determined by GPC.

EXAMPLE 4 Acrylic Acid (80 mole %)/Acryloyl Morpholine (20 mole %)Copolymer

A solution of acrylic acid (40.28 g), acryloyl morpholine (19.72 g) inwater (310 g) was charged into a 1-liter, 4-neck round bottom flaskwhich was equipped with a mechanical stirrer, a thermometer, and acondenser. The solution was heated to 60° C. under nitrogen atmosphere,and ammonium persulfate (3 g) in water (10 g) and sodium bisulfite (9 g)in water (20 g) were added in sequence. The reaction was maintained at70° C. for three hours.

GPC analysis showed the copolymer has a molecular weight of 15,600.

EXAMPLE 5 Acrylic Acid (69 mole %)/Methacrylic Acid (19.2 mole%)/Acryloyl Morpholine (11.8 mole %) Terpolymer

A solution of acrylic acid (30 g), methacrylic acid (10 g), acryloylmorpholine (10 g) in water (260 g) was charged into a 1-liter, 4-neckround bottom reaction flask which was equipped with a mechanicalstirrer, a thermometer, and a condenser. The solution was heated to 65°C. under nitrogen atmosphere. Ammonium persulfate (2.5 g) in water (10g) and sodium bisulfite (7.5 g) in water (20 g) were then added intosolution in sequence. The reaction was maintained at around 65° C. forthree hours.

The molecular weight of the polymer was 19,500 as determined by GPC.

EXAMPLE 6 Acrylic Acid (80 mole %)/Methacryloyl Morpholine (20 mole%)Copolymer

A solution of acrylic acid (42.60 g), methacryloyl morpholine (23.25 g)in water (319.98 g) was charged into a 1-liter 4-neck round bottomreaction flask which was equipped with a mechanical stirrer, athermometer, and a condenser. The solution was heated to 60° C. undernitrogen atmosphere. Ammonium persulfate (3.29 g) in water (10 g) andsodium bisulfite (9.88 g) in water (20 g) were added in sequence. Thereaction was maintained at about 75° C. for three hours.

Analyses showed the sample contained 2.9% residual acrylic acid and themolecular weight of the terpolymer was 3890.

In all the above preparations, the molecular weight of the polymers wasbetween 3,000-150,000.

Testing of the Polymers as Calcium Phosphate Inhibitors

The following test method was used:

Procedure for o-PO₄ Filtration Test (Ca₃)(PO₄)₂ Stabilization Test (Noteall chemicals are reagent except for treatments)

1. Put 300 to 350 ml of DI water in the 600 ml jacketed beakers and letstand with mild stirring until temperature is brought to 150 degrees F.(70 degrees C.) by use of a constant temperature water bath.

2. Put in required ml of stock hardness into jacketed beakers: For 250ppm CaCO₃ use 50 ml or any desired hardness

To make 2 liters of stock solution:

1. Dissolve 7.356 g CaCl₂ 2H₂ O in 800 ml DI H₂ O.

2. Dissolve 6.156 g MgSO₄ 7H₂ O in 800 ml DI H₂ O.

3. Add both solutions to 2 liter volumetric flask and dilute to volume.

4. Shake well.

3. Add sufficient ml of treatment into jacketed beakers while stirring(normally 5 mls for 10 ppm concentration).

4. Add DI water to make 500 ml in jacketed beakers (add water to line onbeaker with stirrer not operating).

5. With stirring, let solutions in beakers equilibriate to 158 degreesF.

6. With stirring, adjust pH to 8.5 with dilute (0.1-0.4N) NaOH.

7. Add 5 ml of 1000 ppm PO₄, pH-8.5 solution to jacketed beakers andwait about 3-5" while stirring.

8. Check pH of solution in beakers and as necessary adjust pH to 8.5±while stirring.

9. Let experiment run at 158 degrees F. with stirring for 4 hours.

10. After 15 minutes, check pH of solutions in beakers and as necessaryadjust pH to pH 8.5±0.1. Also, check pH of solutions every 30 to 45minutes thereafter.

11. After the 4 hours are up, the solution is immediately filteredthrough 0.45 micron filter paper under vacuum. The filtered solution isanalyzed for o-PO₄ using standard procedure and the color is evaluatedin the Spec at 700 nm.

12. The results are reported as percent inhibition calculated by thefollowing formula: ##EQU1## wherein: initial-o-PO₄ =o-PO₄ concentrationin the mixture at the beginning of the experiment.

residual-o-PO₄ =o-PO₄ concentration in the mixture at the end of theexperiment with stabilizer.

blank residual-o-PO₄ =o-PO₄ concentration is the filtrate at the end ofthe experiment with no stabilizer.

Calcium Carbonate Inhibition Test

The apparatus is based upon a Mettler automatic titration system. TheE-px converter was calibrated according to the two buffer-twotemperature method described in the Mettler DK12 E-px converter manual.This calibration allows the equipment to compensate the pH values fortemperature changes over a range of 20°-70° C. The Mettler E-pxconverter output is 100 MV/pH unit. This signal is recorded as theobserved MV change as the titration progresses, and the pH of thetitration breakpoint is most readily obtained from the observed MVvalues. To convert over to true MV changes, multiply the observed MVchange times the decimal equivalent of the value on the d E/d px dial. Astandard pH 7 (pH 6.98 at 60° C.) buffer solution was prepared fromBeckman buffer powder and employed to determine the -1 MV (true)reference point after each titration. A stock hardness solutioncontaining 3600 ppm Ca²⁺ and 2000 ppm Mg²⁺ (equivalent to 20X Synthetic#3 PCT test water) is prepared using CaCl₂.2H₂ O and MgSO₄.7H₂ O. A 2200ppm HCO₃ solution was prepared each day using NaHCO₃, and the 0.1N NaOHtitrant is prepared using an Acculate reagent solution. To a 100 mlvolumetric flask is added the inhibitor, hardness (20 mL), andbicarbonate (20 mL) solutions followed by dilution with distilled water.The solution is swirled gently to mix the reagents, then transferred toa 300 mL jacketed-Pyrex beaker maintained at 60° C. The initial reagentconcentrations are 360 ppm Ca²⁺, 200 ppm Mg²⁺, 440 ppm HCO⁻ ₃. Generallydosage preference curves are obtained using 5, 10, or 15 ppm inhibitoractives (0.5, 1.0, or 1.5 mL of stock solution). The test solution isstirred for 6 min. to allow for temperature equilibraton. Dispensing tipfor the NaOH titrant is placed just above the test solution surface.Next, the pH electrode is lowered as far as possible into the solution,but maintaining clearance above the magnetic stirbar. The titrant isadded at 0.3 mL/min (buret drive unit rate=2) until the strip-chartrecorder indicates a small pH drop has occurred. It should be noted thatmany test solutions exhibit significant turbidity before the actual pHbreakpoint (associated with massive CaCO₃ precipitation) is observed. Ifadditional base was added a second break-point associated with Mg(OH)₂precipitation usually is observed at higher pH values. After thebreakpoint is reached, several mL of 10% HCl is added to the testsolution to dissolve the precipitated CaCO₃. After each titration, thepH electrode is transferred to the standard pH 6.98 (-1 MV, true)buffer, which is maintained at 60° C., and this reference point isindicated on the recorder. After rinsing the jacketed-beaker withdeionized water, a new test solution is added. A calibration check ismade daily by titrating a test sample containing 10 ppm actives Dequest2010* (˜120 MV, true). Relative error of this titration method generallyis ±1% (daily) and ±2% (weeks). Saturation ratios (S.R.) are calculatedusing the method as described by J. C. Westall, et al. entitled,"Mineol, A Computer Program for the Calculation of Chemical EquilibriumComposition of Aqueous Systems," Water Quality Laboratory, Ralph M.Parsons Laboratory for Water Resources and Environmental Engineering,Department of Civil Engineering, Massachusetts Institute of Technology,Technical Note No. 18, Sponsored by EPA Grant No. R- 803738, July 1976,pp. 8-10.

                                      TABLE I    __________________________________________________________________________    Using the above test method, the following additional examples were run:                        % Calcium Phosphate.sup.1                        Inhibition (Polymer Dosage)                                        CaCO.sub.3 Saturation Ratio    Ex.       Polymer Composition                    MW.sup.2                        5 7.5                            10 20 ppm actives                                        10 15 ppm actives    __________________________________________________________________________     7 Sulfonated Styrene-                    18,950                         8                          --                            87 --       Maleic acid (75/25)     8 Acrylic acid-                     7,350                        13                          75                            92 --       Hydroxypropyl acrylate       (75/25)     9 Acrylic acid-                    12,600                        --                          --                            82 84       89 98       Acryloyl morpholine       (70/30)    10 Acrylic acid-                    90,100                        --                          --                            45 77       84 89       Acryloyl morpholine       (70/30)    11 Acrylic acid-                    15,600                        11                          65                            96 --       101                                           126       Acryloyl morpholine       (80/20)    12 Acrylic acid-                    11,200                        --                          --                            97 100       Acryloyl morpholine       (90/10)    13 Acrylic acid-                    19,500                        --                          --                            100                               --       Methacrylic acid-       Acryloyl morpholine       (69/19.2/11.8)    14 Acrylic Acid-                     5,770  16 98       Methacryloyl Morpholine       (80/20)       Blank                            10 10    __________________________________________________________________________     .sup.1 All screening tests were done using water containing 250 ppm of Ca     (as CaCO.sub.3), 125 pm Mg (as CaCO.sub.3) and pH 8.5.     .sup.2 Molecular weights were determined by GPC in aqueous solution using     sulfonated polystyrene standard.     In the above examples, Examples 7 and 8 show prior art calcium phosphate     inhibitors.

I claim:
 1. A method of inhibiting calcium phosphate scales in boilersand industrial cooling systems which comprises treating the waterpresent in these systems with an effective amount of meth-acryloylmorpholine and acryloyl morpholine copolymers which comprises 5-95 molepercent of meth-acryloyl morpholine and 95-5 mole percent of carboxylicacid containing vinyl monomer comprising acrylic acid, methacrylic acid,maleic acid, crotonic acid, and itaconic acid and which polymer has amolecular weight within the range of 1,000-150,000 to inhibit saidcalcium phosphate scales.
 2. A method of inhibiting calcium phosphatescales in boilers and industrial cooling systems which comprisestreating the water present in these systems with an effective amount ofmeth-acryloyl morpholine and acryloyl morpholine copolymers whichcomprises 5-95 mole percent of meth-acryloyl morpholine and 95-5 molepercent of (meth)acrylic acid and which polymer has a molecular weightwithin the range of 1,000-150,000 to inhibit said calcium phosphatescales.